To insert intervertebral prostheses consisting of two prosthesis plates, each to be connected to a respective vertebral body, and of a prosthesis core arranged between these plates, insertion instruments are known (EP-A-333 990) which, at their front end, have two prosthesis holders which each receive a prosthesis plate. The prosthesis holders are connected to one another via a parallel guide which makes it possible initially to bring the prosthesis plates very close to one another, so as to be able more easily to introduce them into the narrow intervertebral space, and then to spread them apart with the adjoining vertebrae in order to be able to insert the prosthesis core between them. Thereafter, the prosthesis holders are moved back toward one another so that the prosthesis plates receive and securely hold the prosthesis core, and the instrument is removed. The known instrument is designed as a forceps which is angled in relation to the direction of the prosthesis holders, which direction is intended to coincide with the median direction of the body. The use of forceps has the disadvantage that the forceps bows which the operating surgeon holds in the hand do not give him an exact picture of the orientation of the front end of the forceps and of the prosthesis to be inserted. A similar instrument (DE-U-201 16 410) is used for introducing and spreading a total prosthesis.
An instrument for introducing a prosthesis is also known (DE-U-299 16 078) which is formed by a lower pair of guide rods and an upper guide rod, these rods being articulated on one another at the rear end and carrying prosthesis holders at their front ends. They form a guide track for a spreader element. When the latter is driven forward between them by means of a toothed rack, it spreads the rod ends apart and at the same time pushes the prosthesis core ahead of it until the latter has reached the desired end position. Thereafter, the spreader element is drawn back in order to bring the prosthesis plates toward the prosthesis core. In this case, the spreading movement is inextricably linked with the introduction of the prosthesis core, so that the spreading operation is not separate from the introduction of the prosthesis core and can be observed only with difficulty.
Instruments whose purpose is only to spread the vertebrae apart are also known. One of these known instruments (DE-U-200 04 812) has, at the front end of an elongate instrument body, two spreader members which are connected by a parallel guide and can be spread via a push rod, extending within the instrument body, and via toggle levers. The push rod is actuated by a forceps-like auxiliary instrument. This arrangement is complicated. In another known instrument (DE-A-198 36 498, FIGS. 5 and 6), two spreader members are connected to one another via a parallel guide and can be spread apart from one another by a transversely extending threaded rod. As this threaded rod can be actuated only with difficulty within the operating site, a design is instead recommended (FIG. 1) in which the spreader members are arranged on a forceps, and a parallel guide is dispensed with. Finally, a spreading forceps is known (EP-A-269935) in which one of two spreader members connected by a parallel guide is connected to one shank of a spreading forceps, while the other shank is articulated on the other spreader member. The spreader members comprise pins which are fitted into bores of the vertebral bodies which are to be spread apart. The vertebral bodies are thereby weakened. The use of a forceps also has the disadvantage that the forceps bows do not give the operating surgeon an exact picture of the orientation of the prosthesis.